Pseudopollen:: Its structure and development in Maxillaria (Orchidaceae)

Histochemical analyses of the pseudopollen of ten species of Maxillaria section Grandiflorae revealed that the main storage product is protein, although starch is usually also present. Lipids are rare in pseudopollen and thus do not seem to play an important role in attracting insects. In Maxillaria sanderiana, pseudopollen is formed by the fragmentation of multicellular, uniseriate trichomes, derived by the repeated division of a single, papilla-like, basal secretory cell that contains well-developed dictyosomes, endoplasmic reticulum and mitochondria. At first, there is continuity of cytoplasm between adjacent component cells of a trichome via plasmodesmata. During maturation, the cytoplasm retracts as the cell volume increases and the plasmodesmata become less obvious. Each component cell of the trichome eventually comprises a large protein body and a small amount of peripheral cytoplasm containing amyloplasts, a few small lipid bodies, mitochondria and a nucleus with nucleolus. Finally, the trichome undergoes fragmentation, forming individual cells or chains of cells of varying lengths. Light microscopy observations indicate a similar sequence in the other species examined. The occurrence of pseudopollen in section Grandiflorae and alliance Splendens may indicate that this character has evolved at least twice in Maxillaria. (C) 2000 Annals of Botany Company.